That one was actually a "badge", as opposed to a decal, and I think Enzo had it glued to the mirror of his Cobra. If you go to a Ferrari dealership, most have either a gift shop or you can go to the parts counter and buy a tie tack or perhaps cufflinks in that size and design. Cut the back off and glue it on to your mirror.
Here is a link to a page that has a 2.75" Ferrari shield for $4.95. Click on the link and then click on "Scuderia Shields" listed on the left side of the page. The 2.75" shield is item #6, I believe. Still pretty large but it's the smallest one I've found so far--
http://www.image-homes.com/Adv.html
Bob